In thermodynamic apparatuses, and in particular in power station condensers, for which the present invention is principally devised, the coefficient of thermal transmission or the reciprocal thereof, i.e. the thermal resistance, is in certain circumstances substantially impaired at the heat exchanger surfaces after a greater or lesser length of operating time by the formation of corrosion products and/or mineral and organic deposits from the cooling water.
This can be counteracted by a continuously operating cleaning system and the addition of corrosion-inhibiting additives to the cooling water, as a result of which a thin, lasting and properly adhering protective layer is formed at the heat exchanger surfaces. Since this layer does of course likewise impair the heat transmission, it is referred to as a protective/dirt layer.
In order to determine whether the overall coefficient of heat transmission still complies with the guaranteed values indicated by the manufacturer, it must be possible for the thermal resistance of this protective/dirt layer to be determined by the customer in the event of a decrease therein after a certain period of operation. A process for the experimental determination of this thermal resistance is described in the ASME publication PTC 12.2, section 5. However, this widespread process is costly, involves a relatively lengthy interruption in operation and is of low accuracy. Accordingly, in the circumstances in which importance is placed on greater accuracy, there is reluctance to carry out the measurement in accordance with this method, since it does not permit a reliable determination of the effect of the factors which are of decisive importance to the coefficient of heat transmission. In this connection, however, the thermal resistance of the protective/dirt layer is only an experimentally unconfirmed assumption, by means of which the difference between the previously calculated and the experimentally determined thermal resistance is to be explained. With such an acceptance test, it is not possible to obtain indications regarding optimization of the elements and other design data of the condenser, such as piping, steam flowrate, cooling water flowrate etc.
The principle of this process according to ASME consists essentially of the following: after shutdown and cooling of the condenser, from each respective set of 2,000 tubes of a bundle of tubes a set of for example seven tubes is selected, consisting of a central tube and six outer tubes, which surround the central tube in the form of a hexagon. The central one of these tubes is replaced by a new one, which has the same new condition as was exhibited by the remaining tubes of the bundle of tubes when the tubes were fitted to the condenser. In order to withdraw the replaced central tube and to introduce the new tube, manhole covers are provided in the two water chambers at the pertinent positions. The seven tubes selected for examination, i.e. the central new tube and the six old tubes surrounding the latter, are connected at their two ends to hoses, which are guided outwardly through the water chambers and the said manholes and are connected to an external cooling water stream. All seven tubes carry cooling water under the same conditions, and steam of the same condition circulates around them. By means of measuring instruments for the mass flow of cooling water and for the inlet and outlet temperature of the cooling water at the inlet and outlet respectively of the cooling water from the selected seven tubes, the mean coefficient of heat transfer of the six outer tubes and the coefficient of heat transfer of the new tube are determined. The ratio of these two values is referred to as the purity factor. For this purity factor, a simple mathematical expression is obtained, which however exhibits the error that the unknown thermal conductivity coefficient of the layer deposited in the old tubes is not included therein. The process is accordingly indeed unreliable, but, as has been mentioned, has nevertheless found widespread application. However, in circumstances in which more accurate results are required, it is not sufficiently reliable, so that a requirement exists for a more accurate test method for the determination of the change in the coefficient of heat transmission of a condenser or similar apparatus having heat exchanger surfaces.